1. Field of the Invention
The present invention relates to a device for protecting a spraying equipment and the surroundings about the spraying equipment mounted in a paper or board machine in connection with a first moving surface of the machine in order to apply a treatment medium onto the first moving surface within a machine-wide application zone, said device comprising a machine-wide protecting cover arranged to enclose the spraying equipment and said application zone and having an internal space, which forms a single chamber in which the spraying equipment is located and which is open towards the first moving surface in connection with said application zone, said protecting cover comprising first and second, machine-wide wall elements, said wall elements extending cross to the machine direction on either side of said application zone, said first wall element being located upstream said application zone, as seen in relation to the direction of movement of the first moving surface onto which the treatment medium is applied, and defines a machine-wide, first gap between itself and the first moving surface, and the second wall element defines a machine-wide, second gap between itself and the first moving surface or a second moving surface.
The invention also relates to a method of protecting a spraying equipment and the surroundings about the spraying equipment mounted in a paper or board machine in connection with a first moving surface of the machine for applying a treatment medium onto the first moving surface within a machine-wide application zone, said application zone being enclosed by a protecting cover so that an internal space of the protecting cover, which space forms a single chamber in which the spraying equipment is located, is open towards the first moving surface so that a machine-wide, first wall element of the protecting cover is located upstream said application zone, as seen in relation to the direction of movement of the first moving surface onto which the treatment medium is applied, and defines a machine-wide, first gap between itself and the first moving surface, and a machine-wide, second wall element of the protecting cover defines a machine-wide, second gap between itself and the first moving surface or a second moving surface.
2. Description of Related Art
An adhesive and protecting coating is applied continuously on the shell surface of a drying cylinder, e.g. a Yankee cylinder, in a tissue machine by a spraying equipment, which includes a plurality of spray nozzles arranged for providing a continuous, machine-wide spray pattern on the shell surface in a position located before the transfer nip that is formed by the Yankee cylinder and a press roll. The spray nozzles emit a spraying liquid which is a mixture of water and special chemicals which are to form said coating. The purpose of the coating is to control the adhesion of the paper web against the Yankee cylinder and to protect the shell surface against wear as well as chemical attacks. The spraying liquid discharged from the spray nozzles has a high pressure, typically 3-5 bar, and hits the shell surface at a high velocity. A portion of the chemicals adheres in the desired way to the hot shell surface, which can have a temperature of e.g. 90-100° C. When the aqueous spraying mixture hits the hot shell surface, a mist is created which contains vapor and non-evaporated water from the spraying liquid, but also residues of chemicals which are not attaching to the shell surface or do not reach the shell surface for some reason, e.g. as a result of the spraying liquid being influenced in an undesired way by lateral forces from the boundary layer air flows which are created along the shell surface as a result of the high speed of rotation of the Yankee cylinder. The chemical-containing mist is carried away from the place of application by local air streams which mainly are produced by the fast production of said water vapor, by convection, by said boundary layer air flows along the shell surface, and by air streams along a clothing running around the adjacent press roll, e.g. a felt or wire running around a suction press roll. The local air streams created in this way and bringing the chemical-containing mist with them, are difficult to control and they can spread out into the surroundings both in the dry end of the tissue machine as well as outside of it, with accompanying problems. Dust from cellulose fibers being released from the paper web and brought along by the surrounding air streams is accumulated in different places of the paper machine. A portion of the chemical-containing mist can get into contact with such dust-coated places, whereby the adhesive chemicals easily adhere to and form a mix with the dust. Furthermore, before the mist reaches these dust-coated places, it can intermix with cellulose fibers occurring in surrounding air streams, when said local air streams get into contact with and intermix with the surrounding air streams. These accumulations of a mixture of said chemicals and dust of cellulose fibers constitute an increased risk of fire with resulting production stops and loss of production, and increase the maintenance costs for cleaning the tissue machine. Furthermore, the chemical-containing air streams can escape from the machine, where the chemicals can be a risk to the machine operators, e.g. undesired inhalation of the chemicals and an annoying or hazardous accumulation of the chemicals on e.g. floor surfaces, so that the floors become slippery and the accumulation has an influence on the environment. There are similar problems also in other places of the described machine and in other paper machines as well as board machines, where a spraying equipment is arranged in connection with a moving surface onto which a coating is applied.
U.S. Pat. No. 6,248,407 describes applicators for applying an atomized coating agent onto a paper web by a spray nozzle. In one embodiment, the applicator comprises a housing having a chamber in which the spray nozzle is located. Steam is supplied to the chamber continuously for creating and maintaining a humid atmosphere inside the chamber in order to prevent the droplets and particles of the coating agent from drying on their way towards the web, thereby maintaining a low viscosity of the coating agent, so that the produced coating attains a smooth surface. Any excess of treatment liquid is drained through an outlet from the chamber. The housing with its chamber for maintaining a humid atmosphere is thus an integral part of the applicator itself. The patent neither describes nor indicates the problems ensuing from a spraying equipment and its surroundings, as discussed above, but is exclusively about creating a humid atmosphere for the coating agent in its application stage.
U.S. Pat. No. 6,203,858 describes applicators for coating a web with a coating agent which first is applied onto a roll encountering the web. The applicators include a spray nozzle and a vacuum chamber enclosing the spray nozzle and possibly an additional chamber. In the latter case the inlet side of the vacuum chamber is closed by a mechanical scraping device. In the first case, i.e. without said additional chamber, the vacuum chamber forms gaps at its inlet and outlet sides through which air is sucked into the vacuum chamber. Via a special internal gap formed by a baffle plate surplus spray mist is sucked into the vacuum chamber and out of it via an outlet, while coating agent attaching onto the insides of the vacuum chamber runs down therefrom and is drawn off through a special outlet in order to be recovered. The fact that air flows into the vacuum chamber both at the inlet side and at the outlet side indicates that a rotation at a low speed is concerned when applying chemicals onto the roll in order to form a coating which subsequently is to be transferred to the web, and that no special measures have been made or disclosed in order to seal the vacuum chamber in relation to the coating-transferring roll, neither in connection therewith for providing and maintaining a controlled supply and removal of air and through-flow of air in the vacuum chamber in connection with the more difficult operating conditions prevailing in a high speed machine. Neither does the patent touch upon the above-discussed problems concerning an unprotected spraying equipment at a hot Yankee cylinder and the environment surrounding it, especially when high speed machines are concerned.
The object of the present invention is to reduce the above-mentioned problems essentially and to create conditions for an improved hygiene both inside and outside the paper or board machine, a reduced fire hazard in the dry end of the paper or board machine as a result of accumulation of dust, such as fibers and fiber parts, and chemicals on the machine stands, an improved working environment for the operating staff and, in addition, an improved efficiency when applying the coating onto a cylinder surface or a supported paper or board web.